1. Field of the Invention
The invention relates to a double-surface printing type printing apparatus, in which a sheet with one surface thereof having been subjected to printing is once reversed, and then the other surface of the sheet is subjected to printing to finish a double-surface printing, and more particularly, to a double-surface printing apparatus, in which a sheet having experienced only one-surface printing and not having experienced any double-surface printing is prevented from being inadvertently taken out.
2. Description of the Related Art
With the miniaturization and cost reduction of a host personal computer, a printing apparatus, in particular, a so-called page printer has been promoted to be reduced in size and cost. Many of the printing apparatuses conventionally used as a printing apparatus and having a function of double-surface printing are mostly expensive large-sized printing apparatuses. Further, to solve this issue, the inventions described in Japanese Patent Unexamined Publication Nos. 255476/1990 and 215646/1990 have been proposed, in which a portion above a paper discharge tray surface in a printing apparatus is used for guiding a sheet to a sheet conveyance path for feeding the same sheet again into a apparatus body.
However, since a portion of a sheet having a first surface (front surface) after printing temporarily comes out of the apparatus, it is feared that some short-tempered users would pull out the sheet before the sheet is put in a sheet reverse conveying condition in order that the sheet be conveyed into the printing apparatus for printing of a second surface (back surface). Therefore, the apparatus is required to be provided with additional abnormality detecting means for detecting the sheet being taken away midway, and several complex arrangements for issuing to a host side a demand to the effect that printing data be fed again.
In addition, if a cover is mounted on a paper discharge tray surface so as to prevent a sheet from being inadvertently taken out during printing, the cover must be opened every time a sheet printed is to be removed, which makes handling inconvenient.
To solve the problem, the invention disclosed in, for example, Japanese Patent Unexamined Publication No. 157212/1998 has been proposed. The proposed invention provides a double-surface printing apparatus, in which in performing a double-surface printing processing, at least a portion of a sheet during the processing is once exposed to a conveyance path accessible from outside the apparatus, and the sheet is again conveyed into the apparatus to be subjected to the double-surface printing processing. The apparatus comprises reconveyance discriminating means for discriminating whether or not the sheet should be reconveyed when the sheet is exposed to the conveyance path, and alarming means for giving an alarm to an operator when the reconveyance discriminating means identifies the exposed sheet being an object of reconveyance. The alarming means includes protective means, which covers the conveyance path to thereby inhibit the exposed sheet from being touched.
Also, an image forming apparatus capable of double-surface printing, disclosed in Japanese Patent Unexamined Publication No. 140733/1995, is also known. The apparatus of the invention comprises feed means provided in a substantially horizontal direction, discharge/load means disposed above the feed means, image forming means and fixing means provided in a conveyance path in a substantially vertical direction, which path connects between the feed means and the discharge/load means, first switching means provided immediately after the fixing means, for switching the sheet to a direction different from a direction toward the discharge/load means, a reconveyance path for reversing the sheet, of which direction has been switched by the first switching means, guiding the sheet substantially vertically downwardly, and guiding the sheet downward below the feed means, reversal conveying means for reversing and conveying the sheet being conveyed along the reconveyance path, and second switching means for guiding the sheet, which is reversed and conveyed by the reversal conveying means, again toward the image forming means. With such arrangement, at the time of double-surface image formation, the first switching means conducts a sheet, of which one surface is formed with an image, to the reconveyance path, and the sheet conveyed along the reconveyance path is reversed and conveyed by the reversal conveying means, and is conducted again to the image forming means by the second switching means for double-surface image formation. In this invention, a sheet entering the reconveyance path is reversed for printing on both surfaces without being exposed to outside of the apparatus, and discharged, so that subsequently discharged, so that there is no fear that a sheet is taken out before the completion of image formation on both surfaces.
Incidentally, with the invention disclosed in the Japanese Patent Unexamined Publication No. 157212/1998, in the event of double-surface printing being specified, a FU paper discharge tray, which serves as a paper discharge tray when a printed surface of a sheet turns upward, is turned to a position to lie above a FD paper discharge tray, which discharge sheets with a surface printed thereof turned downward, to lie above a sheet, which is switched back for reversal. In this case, sheets having been subjected to double-surface printing are discharged to the FD paper discharge tray, and the FU paper discharge tray lies the sheets, while it is possible to take out the sheets by gaining access thereto from between the FD paper discharge tray and the FU paper discharge tray. Therefore, there is the possibility that a user may take out a sheet exposed for reversal with the intention of taking out a sheet having been subjected to printing.
Further, the above Japanese Patent Unexamined Publication No. 140733/1995 discloses the image forming apparatus, in which the reconveyance path disposed in an outer housing is constructed such that a sheet positioned in the reconveyance path cannot be taken out from outside. In the image forming apparatus, a reversal path is disposed in a lower portion of the housing, while it is disposed in an embodiment below a position where a sheet feed cassette is arranged, or disposed on a side of the housing to be in parallel to the reconveyance path. However, if the reversal path is disposed in the lower portion of, or on the side of the housing, the conveyance path becomes lengthy, which cannot but make the apparatus large in size, and cannot meet the demand for miniaturization.
Furthermore, with the image forming apparatus of this type, curl of a sheet is sometimes problematic. Curl of a sheet is caused by the application of heating or pressurizing by rollers of a fixing unit, such that a sheet passed through the fixing unit is normally discharged in a curled condition. There is a fear that when a sheet is taken into a reversal conveying section for printing on a second surface of the sheet, it may cause jamming wrapping in its leading end, which goes ahead, and may cause sheet jamming in the conveyance path after the reversal dependent on the degree of such curl.